Deficiencies in the activities of mitochondrial enzymes have been documented in a number of neurodegenerative disorders, but the precise mechanisms by which the enzyme deficiencies impair mitochondrial functions have not been elucidated. Decreased activity of the alpha- ketoglutarate dehydrogenase complex (KGDHC) in AD is especially notable. KGDHC deficiency has also been described in brain in Parkinson's Disease and Spinocerebellar Ataxia Type 1. This Project will define, in detail, the effects of increasingly severe deficiencies of KGDHC on key mitochondrial functions which are known to be more sensitive indicators of mitochondrial damage than is ATP production. These are intramitochondrial redox metabolism, calcium transport, the permeability transition (PT), and transcription of mitochondrial DNA (mtDNA). Studies of intramitochondrial redox metabolism will test the hypothesis that reduced KGDHC activity impairs intramitochondrial redox metabolism by altering lipoate metabolism. Studies of calcium regulation will test the hypothesis that the effects of reduced KGDHC activity on mitochondrial calcium regulation are mediated by decreased mitochondrial membrane potential (delta psi) and precede overt defects in oxygen consumption on the presence of ADP (state 3 respiration. Studies of the permeability transition (PT) will test the hypothesis that progressive KGDHC inhibition results in reduced resistance of mitochondria to induction of a PT, and that this alteration in resistance to PT is mediated primarily by variations of delta psi and in availability of NADH. Studies of transcription of mtDNA will test the hypothesis that KGDHC deficiency leads to reduced mtDNA transcription, and that the decreased transcription is mediated in part by oxidative stress and reduced oxidative phosphorylation (OXPHOS). These studies will initially utilize rat liver mitochondria, which are a biochemically well-characterized system that allows valid and efficient measurement of the properties of interest. Once the fundamental mechanisms by which increasingly severe KGDHC deficiency impairs mitochondrial function have been clarified in this system, future studies will be extended to mitochondria from neural tissues (which are an experimentally more complex system). These studies offer a new and powerful approach to the development of new diagnostic tests and even new therapies for AD and other neurodegenerative disorders of aging.